Structures of mono-unsaturated triacylglycerols. II. The β2 polymorph

An improved crystal structure model has been established for the β2 polymorph of the symmetric mono‐unsaturated triacylglycerol 1,3‐distearoyl‐2‐oleoylglycerol (SOS) and the equivalent β‐V polymorph of Ivory Coast cocoa butter. In addition, the crystal structures of the β2 polymorphs are reported for the triacylglycerols 1,3‐dipalmitoyl‐2‐oleoylglycerol (POP) and 1‐palmitoyl‐2‐oleoyl‐3‐stearoylglycerol (POS), which are, together with SOS, the major components of cocoa butter, and that of 1‐stearoyl‐2‐oleoyl‐3‐arachidoylglycerol (SOA). The existence of β2‐POS and β2‐SOA has not been previously reported in the literature. All structures have been solved from high‐resolution laboratory or synchrotron powder diffraction data with the direct‐space parallel‐tempering program FOX and refined with the Rietveld module of GSAS. All compounds crystallize in similar monoclinic unit cells (Cc) with very long b axes (> 127 Å). The oleic chains are packed together and sandwiched between saturated chain layers, forming acyl‐chain three‐packs. An analysis of the β2 polymorphs and β1 polymorphs [van Mechelen et al. (2006). Acta Cryst. B62, 1121–1130] shows that they contain the same three‐packs and differ only in the symmetry relation between the three‐packs. The three‐pack build‐up provides an explanation of the mechanism of the phase transition that causes the formation of fat bloom on dark chocolate.